1. Field of the Invention
The present invention relates to a solid fuel cell, to a cermet material, to the process for the preparation of said cermet, and to a method for producing energy using such cell.
2. Description of the Related Art
Solid-oxide fuel cells (SOFCs) convert chemical energy into electrical energy with high efficiency and low emission of pollutants. Although the introduction of a “green energy” might seem an attractive scenario, its implementation is beset with technical and economic difficulties.
The most common anodes materials for solid oxide fuel cells comprise nickel (Ni) cermets (ceramic and metallic composite materials) prepared by high-temperature calcination of NiO and ceramic powders, usually yttria-stabilized zirconia (YSZ) powders. These Ni-cermets perform with H2 fuels and allow internal steam reforming of hydrocarbons if there is sufficient water in the feed to the anode. Because Ni catalyzes the formation of graphite fibers in dry methane, it is necessary to operate anodes at steam/methane ratios greater than 3, as from WO 00/52780 (in the name of Gas Research Institute).
S. J. A. Livermore et al. Journal of Power Sources, vol. 86 (2000), 411-416, refers to a cermet anode for SOFC made of nickel and ceria-gadolinia (CGO). This anode performs at 600° C. using 10% H2/N2 as the fuel.
A. Müller et al., Proc. of the 3rd European Solid Fuel Cell Forum, Nantes France, June 1998, 353-362, relate to a Ni-YSZ anode fuel cell. It is envisaged a degradation of the anode related to microstructural changes occurring during operation. The nickel particles have a mean diameter of about 0.5 μm, and are homogeneously distributed in the anode. After long term operation at high current density and fuel utilization (H2+H2O), the agglomeration of the nickel particles leads to a decrease of the amount of three-phase boundary (TPB), resulting in an increase in the anode losses.
A. C. Müller et al., HTMC IUPAC Jülich 2000 suggest that the degradation described by the previous document could be prevented by a multilayer anode whose divers layers differ in their microstructure to fulfill the locally different requirements for SOFC anodes. In particular, the content of Ni and the Ni particle size should increase from first layer (that in contact with the electrolyte) to last layer, thus increasing electronic conductivity, TEC (Thermal Expansion Coefficient) and porosity. The YSZ content should simultaneously decrease. The cermet samples were prepared by mixing 65-85 mol % NiO powder with YSZ powder and sintering them in air at 1300° C. for 5 hours. The particle size of the metallic portion was 0.5-8 μm.
The use of nickel as the metallic component of a cermet anode is advantageous, but its performance drops in short time, especially when a dry hydrocarbon is the fuel, due to graphite formation.
R. J. Gorte et al., Adv. Mater., 2000, vol. 12, No. 19, 1465-1469, propose to substitute nickel with copper (Cu) in a cermet wherein the ceramic portion is YSZ. Other components, including ceria (CeO2), can be added to the metallic portion. In this configuration the role of CeO2 is mainly to provide catalytic activity for the oxidation of hydrocarbons. As shown in FIG. 4a of this paper, the cell prepared with Cu but without ceria exhibits poor performance at 700° C., especially when methane is used as fuel.
C. Lu et al., High Temperature Materials, Proceedings volume 2002-5, Ed. S. C. Singhal, Pacific Northwest National Laboratory, Richland, Wash., USA, relate to a Cu-SDC (samaria-doped ceria) anode composite cell performing H2 and butene fuels at 700° C.
From the above studies, it results that copper alone cannot be an efficient substitute for nickel as its performance is insufficient, in particular with fuels such as dry hydrocarbons.
H. Kim et al., J. Electrochem. Soc., vol. 149 (3), A247-A250 (2002) examine the use of Cu—Ni alloys as anode component for the direct oxidation of methane in SOFC at about 800° C. The ceramic portion, which in this case is YSZ, is made by casting a tape with graphite pore formers over a green tape of YSZ without pore formers, firing the two-layered tape to about 1500° C. The porous anode layer was then impregnated with an aqueous solution of Ce(NO3)3.H2O and calcinated at about 500° C. to decompose the nitrate ions and form CeO2. After the addition of ceria, the porous layer was impregnated with a mixed, aqueous solution of Cu(NO3)2.H2O and Ni(NO3)2—H2O having the desired Cu:Ni ratio. Finally the wafer was again heated to about 500° C. in air to decompose the nitrates and reduced in flowing H2 at about 900° C.
The Applicant has faced the problem of providing a solid oxide fuel cell which is able to show high efficiency and to maintain its performance over time, particularly in terms of a low overpotential in a wide range of temperatures. Moreover, the fuel cell should be able to show the above characteristics when fed with different fuels. Endurance of performance is particularly important when a dry hydrocarbon is used as fuel, since it tends to form graphite fibers on the metallic portion of the cermet anode, which eventually annihilate the fuel cell activity.